One goal in the area of gene therapy is to develop expression cassettes and vectors that are capable of expressing a nucleic acid molecule, such as an antisense molecule, and/or a polypeptide for extended periods of time within a desired cell, tissue, or organ. Preferably, these cassettes and vectors will not trigger an immune response in the body of a subject. The expression cassettes and vectors will therefore be able to provide a therapeutic dose of a polypeptide or nucleic acid molecule for an extended time period.
Preferably, expression cassettes can be included in any type of vector used in gene therapy, such as vectors that integrate into a host cell genome, and episomal vectors that do not integrate into a host cell genome.